Multiple mutations in the Nav1.4 sodium channel of New Guinean toxic birds provide auto-resistance to deadly batrachotoxin.

Toxicity has evolved multiple times across the tree of life and serves important functions related to hunting, defense, and parasite deterrence. Toxins are either produced in situ by the toxic organism itself, associated symbionts or acquired through the diet. The ability to exploit toxins from external sources requires adaptations that prevent toxic effects on the consumer (auto-resistance). Here we examine genomic adaptations that could facilitate auto-resistance to the diet-acquired potent neurotoxic alkaloid batrachotoxin (BTX) in New Guinean toxic birds. Our work documents two new toxic bird species and shows that toxic birds carry multiple mutations in the SCN4A gene that are under positive selection. This gene encodes the most common vertebrate muscle Nav channel (Nav1.4). Molecular docking results indicate that some of the mutations that are present in the pore-forming segment of the Nav channel, where BTX binds, could reduce its binding affinity. These mutations should therefore prevent the continuous opening of the sodium channels that BTX binding elicits, thereby preventing muscle paralysis and ultimately death. Although these mutations are different from those present in Neotropical Phyllobates poison dart frogs, they occur in the same segments of the Nav1.4 channel. Consequently, in addition to uncovering a greater diversity of toxic bird species than previously known, our work provides an intriguing example of molecular-level convergent adaptations allowing frogs and birds to ingest and use the same neurotoxin. This suggests that genetically modified Nav1.4 channels represent a key adaptation to BTX tolerance and exploitation across vertebrates.